Wheel bearing apparatus is generally known that supports a vehicle wheel with respect to a suspension apparatus and detects rotational speed of a vehicle wheel to control the anti-lock braking system (ABS). Such a bearing apparatus usually includes a sealing apparatus arranged between inner and outer members. The inner and outer members rotate relatively to each other via rolling elements contained between them. The rotational speed detecting apparatus has a magnetic encoder with alternately arranged magnetic poles along its circumference. The magnetic encoder is integrally formed with the sealing apparatus. A rotational speed sensor detects change of magnetic poles of the magnetic encoder caused by rotation of the wheel.
In general, the rotational speed sensor is mounted on a knuckle, forming part of a suspension apparatus, after the wheel bearing apparatus has been mounted on the knuckle. However, a wheel bearing apparatus has been recently proposed where the rotational speed sensor is adapted to be mounted on the wheel bearing apparatus in order to reduce the entire size of the wheel bearing apparatus. This eliminates the complexity of the adjustment of an air gap between the rotational speed sensor and the magnetic encoder.
FIG. 19 is one example of a wheel bearing apparatus. The wheel bearing apparatus includes an inner member 61, an outer member 62 and double row balls 63a, 63b rollably contained between the inner and outer members 61, 62. The inner member 61 has a wheel hub 64 and an inner ring 65 press-fit onto the wheel hub 64, via a predetermined interference.
The wheel hub 64 has a wheel mounting flange 66 on its one end. One inner raceway surface 64a is formed on the outer circumference of the wheel hub 64. A cylindrical portion 64b axially extends from the inner raceway surface 64a, via a shaft portion 64d. 
The inner ring 65 includes the other inner raceway surface 65a on its outer circumference. The inner ring 65 is press-fit onto the cylindrical portion 64b of the wheel hub 64. The inner ring 65 is axially immovably secured by a caulked portion 64c formed by plastically deforming the end of the cylindrical portion 64b. 
The outer member 62 includes a body mounting flange 62c on its outer circumference. The outer member 62 inner circumference includes outer raceway surfaces 62a, 62b. The outer raceway surface 62a opposes the inner raceway surface 64a of the wheel hub 64. The outer raceway surface 62b opposes the inner raceway surface 65a of the inner ring 65. A seal 67 and a sensor cap 68 are mounted in annular openings formed between the inner and outer members 61, 62. The seal 67 and sensor cap 68 prevent leakage of lubricating grease, sealed within the bearing, and the entry of rain water or dust from the outside into the bearing.
The pitch circle diameter PCDo of the outboard-side row balls 63a is set larger than the pitch circle diameter PCDi balls 63b of the inboard-side row balls 63a (PCDo>PCDi). The ball diameter (do) of the outboard-side balls 63a is set smaller than the ball diameter (di) of the inboard-side balls 63a (do<di). In addition, the number (Zo) of the outboard-side balls 63a is set larger than the number (Zi) of the inboard-side balls 63a (Zo>Zi). This makes it possible to increase the bearing rigidity of the outboard-side as compared with that of the inboard-side. Thus, this increases the bearing life.
As shown in FIG. 20, a pulser ring 69 is press-fit onto the inner ring 65. The pulser ring 69 has a support annulus 70 and a magnetic encoder 71 adhered to one side of the support annulus 70, via vulcanized adhesion.
The sensor cap 68 is press-fit into the inboard-side end of the outer member 62 to close the opening. The sensor cap 68 is press-formed from austenitic stainless steel sheet. The sensor cap 68 has a cup configuration and includes a cylindrical fitting portion 68a that is press-fit into the inner circumference of the inboard-side end of the outer member 62. A bottom portion 68c extends radially inward from the fitting portion 68a, via a radially reduced portion 68b, to cover the end opening of the inner member 61.
An elastic member 72 of synthetic rubber, such as NBR (acrylonitric butadiene rubber), is integrally adhered to the outer circumference of the radially reduced portion 68b of the sensor cap 68. The elastic member 72 is adhered to the sensor cap 68 so that it does not project toward the inboard-side from the side of the bottom portion 68c. This prevents the elastic member 72 from interfering with a rotational speed sensor 73. In addition, the elastic member 72 has an annular projection 72a. The projection 72a projects radially outward beyond the fitting portion 68a of the outer member 62. Thus, it is elastically deformed and press-contacts against the inner circumference of the outer member 62 during the press-fitting of the sensor cap 68. This improves the sealability of the fitting portion 68a. 
A circular recess 74 is formed substantially at the center of the bottom portion 68c of the sensor cap 68. A nut 75 is crimped in the circular recess 74. The rotational speed sensor 73 can be secured onto the sensor cap 68. A securing bolt (not shown) is passed into a female thread 75a, via a mounting flange (not shown) of the rotational speed sensor 73. This smoothly secures the rotational speed sensor 73 without deviated torque on the sensor cap 68 during fastening of the securing bolt.
The rotational speed sensor 73 is arranged at a radially outer position of the bottom portion 68c of the sensor cap 68. The sensor 73 opposes the magnetic encoder 71 while abutting against or being closely adjacent the sensor cap 68. The sensor detects wheel rotational speed by detecting variation of the magnetic flux of the magnetic encoder 71. This makes it possible to obtain a desirable air gap. Thus, this improves workability during assembly of the wheel bearing while omitting complex air gap adjustment. Additionally, since the magnetic encoder is closed by the sensor cap 68, it is possible to provide a wheel bearing apparatus integrated with a rotational speed detecting apparatus that can obtain sealability without adversely affecting the detecting performance (see, JP 2011-196425 A).
In prior art wheel bearing apparatus, the sensor cap 68 is integrally formed with the elastic member 72. The elastic member 72 has the annular projection 72a that projects radially outward beyond the outer circumference of the fitting portion 68a of the sensor cap 68. It has an advantage that the annular projection can be elastically deformed and press-contact against the inner circumference of the outer member 62 during fitting of the sensor cap 68. Thus, this improves the sealability of the fitting portion of the sensor cap 68.
However, the prior art wheel bearing apparatus has a disadvantage in that the fitting surface of the outer member 62 receives small linear scratches from the fitting portion 68a of the sensor cap 68 during insertion of the sensor cap 68. Accordingly, the annular projection 72a of the elastic member 72 may be partially damaged during insertion. Thus, the sealability of the fitting portion 68a of the sensor cap 68 would also be impaired.